(a) Field of the Invention
This invention relates to a probe for use in determining fluid flow rates in a duct, by sensing both total pressure and static pressure for the fluid flow so that the fluid flow rate may be deduced from the differential pressure between the total and static pressures obtained. The invention further relates to apparatus for determining fluid flow rates in a duct, which apparatus includes such a fluid flow sensing probe.
(b) Description of the Prior Art
It is well known to use a Pitot head, to determine fluid flow rates. A typical Pitot head has two probes, one (the impact probe) facing the fluid flow so as to permit measurement of the total pressure of the fluid flow and the other (the static probe) facing away from the fluid flow (i.e. facing the direction of fluid flow), so as to permit measurement of the static pressure of the fluid flow. Then, determining the difference between the total and static pressures enables an assessment of the fluid flow rate to be obtained.
If the flow rate of fluid within a duct is to be determined, it has long been established that a single Pitot head mounted within the duct is not sufficient to enable an accurate assessment of the fluid flow rate to be made. This is on account of the fact that the `velocity flow profile` across the duct is likely to be irregular; and in any event the velocity flow profile is unlikely even to be symmetrical about the duct axis unless the flow profile has become fully developed after a relatively long unimpeded straight run. Consequently, in most typical industrial installations, it is necessary to employ considerably more complex equipment than a simple Pitot head, when the flow rate along a duct must be determined.
In order to overcome the problem of an irregular velocity flow profile within a duct, a single Pitot head may be traversed across the duct carrying the fluid, and differential pressure readings taken at closely-spaced intervals during the Pitot head traverse. A suitable averaging computation may then be performed on the plurality of differential pressure readings so obtained. An alternative is to provide a plurality of individual Pitot heads mounted at closely spaced intervals across the duct, and again to perform a suitable computation on the plurality of differential pressure readings obtained therefrom. In either case, the equipment involved inevitably is complex and expensive, and the determination of the fluid flow rate is a tedious and time consuming process, involving lengthy calculations.
In an attempt to overcome the above problems, there have been proposals for a so-called self-averaging Pitot-type probe, where the total pressure is determined by providing a modified form of impact probe in the form of a pipe extending across the duct in which the fluid flow rate is to be determined, the pipe having a plurality of total pressure openings along its length and there being a total pressure measuring tube mounted within the pipe which tube has a single total pressure opening disposed substantially on the duct axis, whereby a single total pressure reading is obtained from the tube. An example of such a pressure sensing probe is to be found in U.S. Pat. No. 3,581,565 (Peter D. Dieterich).
Though probes similar to those described in U.S. Pat. No. 3,581,565 have been found to perform adequately when the fluid flow along a duct has a velocity flow profile which is substantially symmetrical about the duct axis, nevertheless it has been found that errors occur when the flow profile is irregular - and the more irregular the profile, the greater the errors. It has been established by the present inventor that this is on account of the fact that because the total pressure openings in the pipe are across the width of the duct and the flow profile is irregular across the duct, different total pressures are set up at each total pressure opening, so leading to fluid flow currents within the pipe, from one total pressure opening to another. The averaging function of the single total pressure sensing tube consequently is only incompletely established, by virtue of the existence of those currents.